How do I send incoming calls straight to voicemail? A870

If I was getting a call on my old LG VX4500 I could just touch an outside button once to silence the ringer- but the caller kept hearing it ringing until voicemail answered. They'd never know I silenced them. However, if you hit the button twice, it sent them straight to voicemail.

My new A870 lets you silence the ringer with one touch. BUT is there a way to send it straight to voicemail? It apparently does not let you send the caller straight to voicemail. For example, if you had a call coming in but had to make another call first you'd have to wait until the voicemail took over.

Thanks!